A new method for the diagnostics of charged particle beams by employing localized space‐charge waves is presented. The propagation of space‐charge waves in the form of localized perturbations can be measured easily and accurately in experiments. The results carry valuable information about many beam parameters such as the wave speed, the geometry factorgfor the longitudinal space‐charge field, the beam radius, the beam impedance under longitudinal perturbations, etc. The principle of this technique and an experimental example are given in this article.